Masters thesis of engineering study on effects of flowability on steel fiber distribution patterns and mechanical properties of sfrc

STUDY ON EFFECTS OF FLOWABILITY ON STEEL FIBER DISTRIBUTION PATTERNS AND MECHANICAL PROPERTIES OF SFRC A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering MINGLEI ZHAO Master of Engineering School of Civil Environmental and Chemical Engineering College of Science Engineering and Health RMIT University August 2016 I Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed MINGLEI ZHAO 12/08/2016 II ABSTRACT Steel fiber reinforced concrete (SFRC) is a multiple-composite material developed during the early 1970s In SFRC, short steel fibers are randomly distributed in concrete Steel fibers can prevent the development of micro-cracks inside the concrete and reduce the expansion and development of the macro-cracks, thus enhance mechanical performance of SFRC However, there is lack of studies on the influence of flowability of fresh SFRC on the steel fiber distribution patterns and mechanical properties of hardened SFRC In this research, steel fibers made by the thin-plate shearing method are used Standard specimens are cast in which steel fibers are added to the concrete mix The slumps ranging from 80 mm to 200 mm are employed as the parameter to reflect the flowability of SFRC The main research work is as follows: (1) By cutting the specimens in three directions (transverse, horizontal and vertical sections) and quantizing the steel fibers in each section, effects of flowability on steel fiber distribution patterns are assessed Distribution rate, distribution coefficient and orientation coefficient are the three factors used for describing steel fiber distribution patterns in this research Calculated results of these factors of different flowability SFRC are summarized and compared (2) Basic mechanical properties tests including compressive strength, splitting tensile strength and flexural strength tests are conducted for different flowability SFRC The splitting tensile tests along three directions of specimens of SFRC are carried out in view of the different orientation of steel III fibers in these directions Load-deflection curve of flexural toughness test is plotted and analyzed (3) Two commonly used methods, i.e., ASTM C1018 (Standard Test Methods for Flexural Toughness and First Crack Strength of Fiber Reinforced Concrete) method and the Chinese Standard JG/T472-2015 (Steel Fiber Reinforced Concrete), are used to access flexural toughness of SFRC Fracture energy is also calculated (4) Formulas for calculating moment of inertia and flexural stress of flowable SFRC are proposed The results show that an increase of flowability has no influence on the orientation of steel fibers and leads to a decrease of sectional uniformity Steel fibers orientated in a longitudinal direction of higher flowability SFRC tend to precipitate towards the bottom layer of the specimens This resulted in much better flexural performance including flexural toughness and fracture energy This would indicate that, instead of studying the entire cross section, the distribution rate and distribution coefficient of steel fibers in tensile zone of specimen should be considered as the main factor determining flexural performance of SFRC Calculations for bending stiffness and flexural stress based on the distribution rate of high flowability SFRC are recommended Moreover, due to the layering effect of steel fibers, traditional test methods are not suitable for determining basic mechanical properties such as compressive strength, splitting tensile strength and flexural strength of SFRC, which require further investigations IV Key words: steel fiber reinforced concrete (SFRC); orientation of steel fiber; flowability of fresh SFRC; compressive strength; splitting tensile strength; flexural strength; flexural toughness; fracture energy V ACKNOWLEDGEMENTS First and foremost, I would like to take this opportunity to express my profound sense of gratitude and indebtedness to my perspicacious supervisors, Dr Jie Li and Dr David Law, for their enthusiastic and expert guidance, continuous help, encouragement, assistance, rationally-based advice and suggestions as well as the critical comments throughout the research study I would also like to acknowledge the University Library of RMIT for their online data base which allowed me to access all the data required in this investigation Without such service, the completion of this research would have been impossible I would like to thank my fellow post-graduate students and friends, Dr Xinxin Ding, Dr Mingshuang Zhao and Ms Leiyuan Yan for their support and contribution to this research Last but not least, I wish to express my deepest gratitude to my father, Mr Shunbo Zhao and mother, Mrs Fenglan Li, for their financial support, wishes, blessings and love Also I am grateful to my girlfriend, Ms Zhen Yang for her encouragement and understanding VI Table of Content ABSTRACT III ACKNOWLEDGEMENTS VI LIST OF FIGURES X LIST OF TABLES XII NOTATION XIII CHAPTER INTRODUCTION 1.1 General 1.2 Research Objectives 1.3 Thesis Arrangement CHAPTER LITERATURE REVIEW 2.1 General 2.2 Factors Influencing Steel Fiber Distribution Patterns 10 2.2.1 Matrix of Concrete 10 2.2.2 Characteristics of Steel Fiber 10 2.2.3 Volume Fraction of Steel Fiber 11 2.2.4 Workability of Fresh Concrete 11 2.2.5 Casting Approach 12 2.2.6 Boundary Condition 13 2.3 Description of Steel Fiber Distribution in SFRC 13 2.3.1 Distribution Rate/concentration of Steel Fiber 13 2.3.2 Distribution Coefficient/uniformly Distributed Variable of Steel Fiber 14 2.3.3 Orientation Coefficient of Steel Fiber 14 2.4 Relationship between Steel Fiber Distribution Patterns and Mechanical Properties of SFRC 15 2.4.1 Distribution Rate/Concentration of Steel Fiber 16 2.4.2 Distribution Coefficient /Uniformly Distributed Variable of Steel Fiber 17 VII 2.4.3 Orientation Coefficient of Steel Fiber 18 2.5 Issues Remaining of Flowable SFRC 18 2.6 Research Questions and Assumptions 19 2.6.1 Research Questions 19 2.6.2 Assumptions 20 2.7 Conclusion 22 CHAPTER EXPERIMENTAL DESIGN 23 3.1 General 23 3.2 Raw Material Tests 23 3.3 Mix Design 25 3.4 Specimens Preparation 26 3.5 Curing of Specimens 28 3.6 Cutting Specimens for Steel Fiber Distribution Patterns Analysis 29 3.7 Mechanical Properties Tests 31 3.7.1 Compressive Strength Test 31 3.7.2 Splitting Tensile Strength Test 31 3.7.3 Flexural Strength Test 32 CHAPTER EVALUATION OF STEEL FIBER DISTRIBUTION PATTERNS 33 4.1 General 33 4.2 Distribution and Orientation of Steel Fibers 33 4.3 Conclusion 42 CHAPTER MECHANICAL PROPERTIES OF SFRC AND THEIR CORRELATION WITH STEEL FIBER DISTRIBUTION PATTERNS 43 5.1 General 43 5.2 Strength of SFRC 43 5.3 Evaluation of Flexural Performance of SFRC 46 5.3.1 Accessing Flexural Toughness through ASTM C1018 Standard 46 5.3.2 Accessing Flexural Toughness by using JG/T 472-2015 Standard 49 VIII 5.3.3 Fracture Energy (Ge,p) 53 5.4 Analysis on Pre-peak-load Performance of SFRC 53 5.4.1 Change in Bending Stiffness (B) 53 5.4.2 Change in Modulus of Elasticity (E) of SFRC 54 5.4.3 Change in Moment of Inertia (I0) 58 5 Post-peak-load Performance 59 5.6 Conclusion 60 CHAPTER CONCLUSION AND RECOMMENDATION 62 6.1 Conclusion 62 6.2 Recommendations for Future Studies 64 REFERENCE 65 IX LIST OF FIGURES Figure Title Page 1-1 Steel Fiber: Cold-Drawn Wire with Hooked Ends 1-2 Steel fiber: Cut Sheet Type with Enlarged Ends (left) or Indentations (right) 1-3 Steel Fiber: Milling Type with Deformed Shape 2-1 Simulation of Steel Fiber Distribution Patterns of Different Flowability 20 SFRC Discussed in Scenario 2-2 Simulation of Steel Fiber Distribution Patterns of Different Flowability 21 SFRC Discussed in Scenario 3-1 Sample of Fiber Used 24 3-2 Machine Used for Blending 27 3-3 Slump Tests of Fresh Concrete Mixture 27 3-4 Vibration of Specimens 28 3-5 Curing of Specimens 29 3-6 Simulation of Cutting Orientation of The Specimens 30 3-7 Gridding of Section Using AutoCAD 30 3-8 Photos of Cut Specimens 30 3-9 Compressive Strength Test 31 3-10 Loading on Specimens for Splitting Tensile Strength 32 3-11 Flexural Strength Test 32 4-1 Distribution Rate of Steel Fibers Versus Layers of Specimens of Different 34 Flowability SFRC 4-2 Transverse Section of Different Flowability SFRC 36 4-3 Vertical Section of Different Flowability SFRC 38 4-4 Horizontal Section of Different Flowability SFRC 40 5-1 Simulation of Cross Section of Splitting Tensile Strength Test 46 X 𝐴0 = the aspect sectional area, 𝐴1 = the additional sectional area by modulus of elasticity of layer 1, 𝐴2 = the additional sectional area by modulus of elasticity of layer 1, 𝐴3 = the additional sectional area by modulus of elasticity of layer 1, 𝐴4 = the additional sectional area by modulus of elasticity of layer 4, 𝑦0 = the aspect neutral axis, 𝐼0 = the altered moment of inertia before crack-elongation Fig 5-5 Simulation of SFRC Stiffness With the calculated moment of inertia 𝐼0 and the modulus of elasticity of concrete 𝐸C , bending stiffness of SFRC 𝐵 before its ultimate flexural strength is calculable using equation (18) In addition, moment of elastic resistance 𝑊0 is usually used to represent the capability of flexural resistance of the section, which can be expressed as: 𝑊0 = 𝐼0 /(ℎ − 𝑦0 ) (26) where, 𝑊0 is the moment of elastic resistance of aspect section area 𝐴0 to the edge of tensile section The ultimate flexural stress 𝜎 of SFRC can be calculated accordingly: 𝜎 = 𝑀𝑐𝑟 /𝑊0 56 (27) 𝑀𝑐𝑟 = 𝑃𝐿 (28) where, 𝑀𝑐𝑟 is the bending moment The calculated results are shown in Table 5-5 As the modulus of elasticity of steel fiber is higher than concrete, from equation (20), a layer with higher steel fiber distribution rate will have a higher modulus of elasticity With the increase of flowability, steel fibers tended to distribute in lower layers of specimens, resulting in the modulus of elasticity of lower layers being higher than those of upper layers Theoretically, SFRC with higher flowability should have better bending stiffness However, as the flowability of 80mm and 120 mm slump SFRC in this experiment was not able to allow steel fibers to freely flow and precipitate in fresh concrete, the layering effect of steel fibers was not obvious Comparing the bending stiffness, there is little difference among each group, which is the reason why the slope of the load-deflection curve before maximum flexural load appears similar On the other hand, the neutral axis tends to move towards tensile section with an increase of flowability, leading to an increase in the moment of elastic resistance From equation (27) and (28), when subjected to the same load, higher flowability SFRC will have a higher moment of elastic resistance and a less flexural stress Both the tensile stress of the steel fibers and the bond stress among the steel fibers and cement will be less than those of lower flowability SFRC Elongation and expansion of cracks could be well controlled, which will result in better ductility and post-crack flexural performance In addition, there are differences in the calculated results of flexural properties whether considering the effects of steel fibers distribution patterns or not The influence of steel fibers on the flexural properties of higher flowability SFRC should not be ignored and require further research 57 Table 5-5 Data of Flexural Resistance of SFRC Slump I0 (mm4) B (N.mm2) (mm) 80 4.374E+07 1.422E+12 120 4.368E+07 1.420E+12 160 4.356E+07 1.416E+12 200 4.369E+07 1.420E+12 Reference* 4.219E+07 1.371E+12 Note: * represent the calculated results without properties σ (MPa) W0 (mm3) fftm (MPa) 5.824E+05 3.128 3.239 5.827E+05 2.927 3.032 5.832E+05 3.028 3.140 5.872E+05 2.986 3.117 5.625E+05 considering effects of steel fiber on SFRC 5.4.3 Change in Moment of Inertia (I0) This is another way to determine the altered moment of inertial Only in this way, the modulus of elasticity is considered unchanged 𝑏ℎ 2 𝑦0 = ( 𝐼0 = 𝑏𝑦0 3 + where, 𝛼e + 𝛼e = 𝐸s /𝐸c (29) 𝐴0 = 𝑏ℎ + (𝛼e − 1)(𝐴1 + 𝐴2 + 𝐴3 + 𝐴4 ) (30) (𝛼e −1)𝐴1 ℎ 𝑏(ℎ−𝑦0 )3 + 3(𝛼e −1)𝐴2 ℎ ℎ + 5(𝛼e −1)𝐴3 ℎ + 7(𝛼e −1)𝐴4 ℎ )/𝐴0 3ℎ + (𝛼e − 1)𝐴1 ( − 𝑦0 )2 + (𝛼e − 1)𝐴2 ( 5ℎ +(𝛼e − 1)𝐴3 ( 7ℎ − 𝑦0 )2 + (𝛼e − 1)𝐴4 ( − 𝑦0 )2 − 𝑦0 )2 (31) (32) = the ratio of the modulus of elasticity of steel fiber to the modulus of elasticity of concrete 58 5 Post-peak-load Performance When the bottom layer steel fibers could no longer provide enough flexural resistance, specimens reached their ultimate flexural strength Cracks elongated and expanded deeper and wider in the beam, and steel fibers in subsequent layers successively jointed in flexural resistance Flexural strength of SFRC decreased with the decrease of effective depth of crack section As shown in Fig 5-6, steel fibers are dispersed more uniformly and closely in the lower layers of higher flowability specimens When one layer of steel fibers was not able to provide enough resistance to the load the next layer of steel fibers could participate immediately, which prevented the rapid development of cracks and produced a much smoother and flatter load-deflection curve In lower flowability specimens, steel fibers were scatted throughout the whole section with larger spaces between each layer, which led to discontinuous fiber-flexural-resistance and a sudden drop of load after the deflection reached mm 𝑅e,k values of 80 mm slump SFRC decrease much more rapidly than that of higher flowability SFRC (a) Simulation of crack elongation and expansion 59 (b) Comparison of steel fiber crack-bridging effect of different flowability SFRC Fig 5-6 Crack Elongation and Expansion Mechanism of SFRC 5.6 Conclusion The mechanisms of traditional splitting tensile, compressive and flexural strength tests does not consider steel fibers flowing and layering phenomenon of high flowability SFRC These methods are deemed not suitable for measuring strength of high flowability SFRC Moreover, to meet the steel fiber layering effect of high flowability SFRC, all tests should be loaded on the cast surface of specimens Suitable test methods should be investigated in order to properly measure mechanical properties of SFRC Both ASTM C1018 and JG/T472-2015 methods were used in this study to assess flexural toughness of SFRC Due to the difficulty of determining first crack deflection, ASTM C1018 indices are considered less suitable for reflecting flexural performance of SFRC While analysing with the JG/T472-2015 method, even though the ultimate splitting and flexural strengths of each flowability SFRC was about the same, SFRC with higher flowability showed much better flexural performance and load-keeping capability These advantages are attributed to the layering effect of transverse section steel fibers as it enhances the crack-bridging capability of the tensile section when 60 loading is in displacement control Steel fibers in lower layers efficiently controlled the elongation and expansion process of cracks and prevented a sudden drop of load As the steel fibers in higher flowability SFRC tended to sink towards the lower layer of specimen, the moment of initial and modulus of elasticity of high flowability SFRC were different with that of lower flowability SFRC Calculations of moment of inertia and flexural stress of flowable SFRC should consider the influence of steel fiber layering The tendency of bending stiffness and moment of elastic resistance increasing with flowability should be more obvious in higher flowability SFRC 61 CHAPTER CONCLUSION AND RECOMMENDATION 6.1 Conclusion The following conclusion can be drawn from the research study: (1) Flowability has an influence on the layering effect of the steel fibers orientated along longitudinal and transverse direction Steel fibers orientated along longitudinal and transverse direction of 80 mm and 120 mm slump SFRC not precipitate as those of 160 mm and 200 mm slump SFRC The steel fibers in 120 mm slump SFRC show better homogenous distribution and steel fibers in 200 mm slump SFRC sink towards the bottom of the specimens The distribution coefficient of steel fibers in each section of 120 mm slump SFRC is the highest in all trials where that of 200 mm slump SFRC was the lowest However, the coefficient of the bottom layer of the transverse section of 200 mm slump SFRC is 0.76, which indicated good uniformity in the tensile zone (2) Flowability does not affect the orientation of steel fibers Aggregate size is hypothesized as the main factor influencing the orientation of steel fibers, as large coarse aggregate restricts orientation and dispersion of fibers (3) The traditional splitting tensile and compressive strength tests are conducted on small cubic specimen, whose mechanisms does not consider steel fibers flowing and layering phenomenon of high flowability SFRC As distribution patterns of steel fibers clearly had influence on strength of SFRC, results of traditional tests methods not reflect real situation of strength of high flowability SFRC Similarly, as traditional flexural strength test is conducted on small beam, whose boundaries limit 62 steel fibers to flow along transverse direction It is deemed not suitable for measuring flexural strength of slab-like members Moreover, to meet the steel fiber layering effect of high flowability SFRC, all tests should be loaded on the cast surface of specimens Suitable test methods should be investigated in order to properly measure mechanical properties of SFRC (4) Both ASTM C1018 and JG/T472-2015 methods were used in this study to assess flexural toughness of SFRC Due to the difficulty of determining first crack deflection, ASTM C1018 indices are considered less suitable for reflecting flexural performance of SFRC While analysing with the JG/T472-2015 method, even though the ultimate splitting and flexural strengths of each flowability SFRC was about the same, SFRC with higher flowability showed much better flexural performance and load-keeping capability, especially after the peak-load When the slump increased from 80 mm to 200 mm, flexural toughness increased up 48% With the increase of deflection, flexural toughness of 80 mm slump specimen decreased up to 77%, where flexural toughness of 200 mm slump specimen decreased only 24% Moreover, when the deflection of specimens reached mm, the fracture energy of 200 mm slump SFRC was 41% higher than 80 mm slump SFRC These advantages are attributed to the layering effect of transverse section steel fibers as it enhances the crack-bridging capability of the tensile section when loading is in displacement control Steel fibers in lower layers efficiently controlled the elongation and expansion process of cracks and prevented a sudden drop of load When the volume fraction of steel fibers remains unchanged, increasing the flowability can be a way to improve flexural performance of SFRC (5) As the steel fibers in higher flowability SFRC tended to sink towards the lower layer of specimen, the moment of initial and modulus of elasticity of high flowability SFRC were different with that of lower flowability SFRC Calculations of moment of inertia and flexural stress of flowable SFRC should consider the influence of steel fiber layering The tendency of bending stiffness and moment of elastic 63 resistance increasing with flowability should be more obvious in higher flowability SFRC 6.2 Recommendations for Future Studies From the conclusion of the research, it can be seen that it is necessary to future study the influence of flowability on the performance of flowable SFRC (1) Current test methods for determining mechanical properties of SFRC are recognized as partially unsuitable The development of new test method is required to accurately determine the performance of SFRC (2) Vibration time can have a large impact on the distribution of the steel fibers, especially when the slump of SFRC is above 160 mm Over-vibration will lead the steel fibers to sink towards the bottom of specimens Control of the vibration time could be a key to control the distribution of the steel fibers (3) Although formulas for calculating mechanical performance of SFRC are proposed in this research, there is not sufficient data to justify whether these formulas can be used for all situation Experiment should be carried out to provide data for verifying the formulas (4) It is quite clear that the change in flowability will influence significantly on the mechanical properties of SFRC However, with the data gathered from this research, it is not possible to propose a formula 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What are the effects of steel fiber distribution patterns on the SFRC mechanical properties? (3) What is the relationship between steel fiber distribution patterns and SFRC mechanical properties? ... mechanical performance of SFRC However, there is lack of studies on the influence of flowability of fresh SFRC on the steel fiber distribution patterns and mechanical properties of hardened SFRC